Z01 HD 001123 LCE and Z01 HD 001124 LCE are companion projects that together investigate auditory communication in primates. The overall goal of these studies is to provide a comprehensive understanding of primate auditory communication in terms of development, neural mechanisms, endocrine factors, and social context. Two non-human primates, the squirrel monkey and common marmoset, are the main subjects of study, with additional data collected from other species where appropriate. Prior work in this project using brain lesioning methodology has shown that production of sounds that are the functional and acoustic equivalents of cry sounds in human infants are mediated by limbic cortex located along the anterior midline of the frontal lobe of the cerebral cortex, and that single neural elements in the auditory cortex (superior temporal gyrus) are particularly responsive to subtle differences in the acoustic structure of species-specific vocalizations, suggesting an important role in mediating individual differences (vocal signatures). A new initiative in the laboratory employs immunocytochemistry to identify the neurons activated in crying infants and in adults hearing infant cries. A study of c-fos expression in infant marmoset monkey brains was conducted. The paradigm involved separating individuals for 30 minutes, recording the cry sounds made during this separation, returning them to their home cage for 1 hour, then euthanizing them and processing their fixed brains for Fos using immunocytochemistry. Of particular note is that regions found to be strongly labeled in adult monkeys were also strongly labeled in infants. In particular, the anterior cingulate gyrus, midline cortex in the frontal lobe, had numerous labelled cells even at 1 month of age. These results indicate that c-fos immunocytochemistry is a method that can demonstrate functional involvement of brain regions also active in adults using the same behavioral paradigm. A brain atlas developed in this laboratory in collaboration with investigators in the NINDS Laboratory of Functional and Metabolic Imaging provided the means to identify the specific structures containing large numbers of Fos. A new study this year used similar methods to identify the neurons activated in adult listeners upon hearing infant cries. Preliminary analysis indicates that structures activated in humans listening to infant cries are also activated in marmoset monkeys upon hearing the cry sounds of marmoset infants.